The present invention generally relates to the field of communication systems and more particularly, to estimating the time and frequency response of at least one desired signal received by at least one antenna and reducing the computational complexity of estimating the time and frequency response of at least one desired signal received by at least one antenna, interpolating the time and frequency response of at least one desired signal received by at least one antenna, and predicting the time and frequency response of at least one desired signal received by at least one antenna.
In a wireless communication system, a major design challenge is to maximize system capacity and performance in the presence of interference and a time-varying multipath channel. Multipath propagation is caused by the transmitted signal reflecting off objects near the transmitter and receiver and arriving at the receiver over multiple paths. Interference in a communication system can come from a variety of sources depending on the particular system deployment. Interference and multipath are major factors that limit the achievable performance and capacity of a communication system because both effects interfere with the ability of a communication receiver to properly decode the transmitted data.
In a multipath propagation channel, the transmitted signal propagates to the receiver over a finite number Lp of propagation paths, where each path has an associated time delay and complex gain. In such a channel, the communication receiver receives the superposition of Lp delayed, attenuated, and phase-shifted copies of the transmitted signal. The number of paths Lp and their time delays and phase shifts depends on the physical location of the various scattering objects (such as buildings, automobiles, and trees) in the immediate vicinity of the transmitter and receiver. The complex attenuation (magnitude and phase) of each path depends on the length of each path as well as the material composition of any scatterers or reflectors encountered along the path.
The presence of multipath can severely distort the received signal. In a multipath environment, the multiple copies of the transmitted signal can interfere constructively in some portions of the occupied bandwidth. In other portions of the occupied bandwidth, the multiple copies can interfere destructively at the receiver. This interference causes unwanted variations in the received signal strength over the bandwidth occupied by the signal. Furthermore, if the difference in the path delays of the various propagation paths is significantly greater than the duration of a transmitted information symbol, then intersymbol interference is present at the receiver. When intersymbol interference is present, the received signal is corrupted by prior transmitted symbols propagating over paths having delays relative to the shortest path that are longer than the duration of an information symbol. The demodulation process (the process of determining which information symbol was transmitted) becomes difficult in the presence of intersymbol interference.
In a mobile wireless communication system, the complex attenuation of each of the multipath components of the received signal becomes a time-varying function of the transmitter""s path and speed throughout the scattering field local to the transmitter""s position. The transmitter""s motion causes the received signal strength at a particular portion of the occupied bandwidth to vary as time progresses. In a mobile multipath channel, the overall channel response not only varies across the occupied bandwidth of the signal, but also across time as well.
In addition to multipath, interference is another system component that limits the performance of a communication system. If the system is deployed in an unlicensed band, then interference can be generated by other users of the band. In a cellular system employing frequency reuse, co-channel interference can be generated by transmitters in another cell that is allocated the same set of frequency channels. Frequency reuse is the practice of assigning the same frequency channels to multiple users of the allocated spectrum.
Many cellular communication systems employ the technique of frequency reuse in order to maximize the utilization of the frequency spectrum allocated to a wide-area system deployment. In a cellular system, a large geographical area is divided into smaller regions called cells, where each cell is served by a single base station operating on an assigned set of frequency channels. Within each cell, multiple subscriber devices are allowed to communicate with the base station on the frequency channels assigned to that cell. The concept of frequency reuse involves allocating different sets of frequency channels to the cells belonging to a particular group and then reusing the same sets of frequencies to the cells belonging to another group of cells.
The reuse factor of a cellular system is defined to be the minimum distance between two cells that are allocated the same set of frequency channels divided by the radius of a cell. A cellular system employing a large reuse factor does not utilize the allocated spectrum as efficiently as a cellular system employing a smaller reuse factor. However, the level of co-channel interference received by a receiver in the cellular system is directly dependent on the reuse factor. Reducing the reuse factor while maintaining the same transmit power tends to increase the level of co-channel interference experienced by a receiver. To better utilize the available spectrum, it would be advantageous to be able to suppress the effects of co-channel interference.
To suppress co-channel interference and equalize the received signals, it is in many cases necessary to estimate and track the time-varying frequency response of the propagation channel. Algorithms known in the art that do not adequately track the time-varying frequency response of the received signal will exhibit poor interference suppression performance and poor equalization performance.
Another technique for increasing the utilization of the frequency spectrum in a wireless communication system is to allocate multiple subscriber devices within a cell to the same time-frequency resources at the same time. Such a technique is known in the art as Spatial Division Multiple Access (SDMA). Many algorithms known in the art that are designed to enable SDMA must be capable of tracking the subscriber devices"" time-frequency variations that are caused by a time-varying multipath channel. Failure to track these variations will reduce the ability of the receiver to demodulate the signals transmitted by the multiple subscriber devices.
One final problem faced in a communications receiver is the amount of operations (e.g., complex multiplies) that its processors can perform per second. This is especially important in mobile receivers where battery power must be conserved (i.e., as a typical rule, the more computations per second required, the more power must be consumed by the receiver""s embedded processors). Even in non-mobile receivers such as base stations, the savings in operations per second can be applied to other operations such as coding or antenna array combining algorithms.
Thus, there is a need for a method and device for estimating the time and frequency response of at least one transmitted signal received on at least one receive antenna. In addition, the method and device for estimating the time and frequency response should also perform the estimation with the fewest operations per second. This invention provides a method for reducing the number of operations per second required by standard channel estimators.
In certain communication applications, there is a need to interpolate between two or more channel estimates. For example, an existing channel estimation can be employed to find the time-domain channel response at three different times. The method and device in this invention then can be used to find the time-domain channel estimates for any time in between the channel estimates found with the existing channel estimation technique.
In other communication applications there may be a need to predict the channel beyond the time or frequency where the existing estimator operates. For example if a communication system has a pilot block followed by data, the method and device in this invention can be used to predict what the channel will be at the data symbols based on the existing channel estimates on the pilot block.